An apparatus of the kind referred to has been proposed, in which gas is withdrawn from the container and cooled and then directly re-circulated in the container for cooling the powder formed. In this known apparatus, the powder collected in the lower part of the container can have such a high temperature that, upon contact with air, it adsorbs such an amount of harmful gases that the quality of the powder may become deteriorated to an unacceptable extent. This involves complications when withdrawing the powder from the container, which either must be performed in an inert atmosphere, or the powder must be allowed to cool before it is withdrawn from the container. However, a compact mass of powder cools slowly and is difficult to cool.